Tomograph acquisition apparatus having a pair of rotatable scintillation detectors which form detection fields at an angle of inclination relative to each other

ABSTRACT

To acquire tomographs, two detectors ( 1, 2 ) are secured together in a support structure ( 3 ) in such a manner that their detection fields are inclined relative to each other. The angle of inclination between them is preferably 90°. This pair of detectors is then placed in such a manner that the plane ( 9 ) bisecting the two detection fields includes the axis of rotation ( 4 ) around which the pair of detectors is to rotate to perform tomography on a subject. For fat subjects, the pair is moved away ( 13 ) from the axis, and for thin subjects it is moved towards it. It is shown that to avoid detector displacement interfering with tomography computation, it suffices merely to change computation parameters in the reconstruction algorithm.

BACKGROUND OF THE INVENTION

The present invention relates to tomograph acquisition apparatus usable,in particular, in the medical field. It relates essentially to acquiringtomographs with scintigraphy detectors that are easy to use.

The principles of scintigraphy detectors is known in nuclear medicine.They are as follows. A radioactive marker, generally technetium, isinjected into a patient. As a function of its nature, the marker isdistributed from its point of injection into various portions of thepatient's body. In the patient's body, the marker is to be found in theorgan under investigation and it reveals the function thereof. Themarker produces gamma photons. After passing through a collimator, thegamma photons are detected by a scintillator crystal, whence the term“scintigraphy detector”. The crystal transforms the gamma photons intolight photons. The light photons are in turn detected by photomultipliertubes placed looking at the scintillator. The currents that flow throughthe photomultiplier tubes are in the form of pulses and are a functionof the magnitude of the scintillation produced. These currents areapplied to a resistance matrix. The resistance matrix outputs “locating”pulses that specify the position at which scintillation took placefacing the photomultiplier tubes.

A counter unit connected to the output from the resistance matrix servesto sum the number of such pulses occurring at each location on thescintillator. It is then possible to create an image representative ofthe activity of the marker in the body by attributing brightness to eachimage point as a function of the number of strikes that have beencounted for each of said locations. Such a method is known in the stateof the art under the name “Anger” method. With very fast scintigraphycameras, e.g. capable of counting up to 200,000 strikes per second, animage constituting a projection of a portion of the human body can begenerated in about 30 seconds.

The detector is mounted in a rotary assembly called a gamma camera whichalso serves to aim the detector. If the detector is aimed in differentdirections relative to the body, multiple images can be acquired underthe same conditions. By acquiring a sufficient number of images fordifferent aiming directions of the detector, the set of image signalscan be subjected to processing suitable for obtaining tomographs of thebody by algorithmic reconstruction. Given that the accuracy of suchtomographic images increases with the number of projection images, itcan be seen that such a method leads to periods of examination that arerelatively long.

Proposals have already been made to remedy this problem by constructinggamma cameras provided with two, three, or even more detectors. Undersuch circumstances, the duration of an examination is reduced,substantially pro rata the number of detectors.

However, another problem arises. To obtain projection images, andconsequently tomographs, that have very good resolution, it is necessaryfor the detectors to be placed as close as possible to the body.Unfortunately, patients to be examined are not all the same size, someare fat, others are thin. In addition, depending on the examinationbeing performed, it may be necessary for the detectors to be at variousdifferent distances from the body. For example, an examination aroundthe belly requires the detectors to be at a different distance from thepatient than an examination around the head, since the diameter of thehead is smaller. A known way of solving this problem is to mount thedetectors on telescopic arms and to move the detectors initially asclose as possible to the patient. During an examination, the detectortravels around a circle whose diameter depends on said distance from thepatient. A device of this kind is for example depicted in U.S. Pat. No.4,368,389.

The drawback of such a telescopic mechanism is that it is twice ascomplex when there are two detectors instead of only one, and so on. Thetelescopic mechanism is itself mounted on a rotary assembly enabling thedetectors to be pointed in different image-taking directions.

SUMMARY OF THE INVENTION

An object of the invention is to remedy these drawbacks firstly bytaking account of the need to place more than one detector on the rotaryassembly in order to accelerate acquisition, and secondly to simplifythe handling of the various detectors. To solve these problems, theinvention begins by securing the two detectors to each other and also bygiving them a certain angle of inclination relative to each other.

Thus, any one detector has a substantially plane detection surfaceconstituting its detection field. At present, such detection fields arerectangular in shape. They have a length and they have a width. In theinvention, two detectors are placed against each other so that thenormals to the centres of their detection fields intersect, and so thatthese detection fields are adjacent to each other along one side of eachof them, which sides are called “lengths”. In the following explanation,the adjacent sides are called lengths, but that does not mean that thedetection field is necessarily longer in that direction than it is alonga direction at rightangles.

It is preferable for the normals to intersect at an angle of 90°.However, this is not essential and the angle could be acute or obtuse.The two detectors are thus secured to each other in this configurationin such a manner that the bisector plane including the point ofintersection of the normals and the adjacent lengths of the twodetectors also contains the axis of rotation of the tomography.

With small patients, the assembly is displaced radially towards the axisof rotation. The corner formed by the two detectors can thus be movedcloser to a small patient on the axis of the machine. Alternatively thecorner can be moved further away when examining a fat patient, likewiseon the axis of the machine. This simplifies the displacement mechanics.

However, by acting in this way, the information acquired duringprojection is not properly situated since the axis of rotation of themachine does not necessarily pass through the point of intersection ofthe normals to the detection fields. To be able to use the samereconstruction algorithms, it is therefore necessary to transform theimage signal processing parameters as a function of the distance betweenthe pair of detectors and the axis of rotation. It is shown that exactlythe same algorithms can be used to reconstitute tomography images, whilealso obtaining a substantial saving on the mechanical equipment which ismuch simpler.

The invention thus provides an apparatus for acquiring tomographs of asubject, the apparatus comprising a pair of plane scintillationdetectors carried by a support rotating about an axis of rotation, andconnected to an image processor, the apparatus being characterised inthat:

the detection fields of these two detectors are inclined at an anglerelative to each other;

the bisector plane bisecting the angle formed between these detectionfields includes the axis of rotation;

and in that the apparatus includes:

means for displacing the pair of detectors together relative to thesubject in a direction which is radial to the axis of rotation; and

modification means for modifying an effective detection field of thesedetectors as a function of said displacement.

The invention will be better understood on reading the followingdescription and examining the accompanying drawings. The drawings aregiven merely by way of indication and do not limit the invention in anyway. In the drawings:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of acquisition apparatus of the invention;

FIG. 2 is a section through the mechanism for rotating the detectors andmoving them relative to the axis of rotation;

FIG. 3 is a perspective diagram of a mechanical detail enabling thedetectors and their counterweights to be moved simultaneously;

FIG. 4 is a block diagram of the set of means implemented by theinvention; and

FIGS. 5a to 5 c are diagrams of the diameters that can be fullyreconstructed for three different sizes of patient.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows tomography acquisition apparatus of the invention. Itincludes a pair of plane detectors 1 and 2 carried together by a commonsupport 3. The support 3 rotates about an axis of rotation 4. The axisof rotation lies slightly above the top surface 5 of the bed 6 thatcarries a patient being examined by the machine. By acting in this way,the axis 4 is caused to lie substantially in the middle of the patient.Both of the detectors 1 and 2 are connected to an image processingapparatus that is explained below. The detection fields 7 and 8 of thedetectors 1 and 2 respectively are at an angle relative to each other.Their adjacent edges, named length, are spaced apart from each other, inone example with a 2 cm space, but preferably this space is less that 8or 10 cm.

In the example shown they are inclined at an angle of substantially 90°to each other and their respective centre-of-field normals 9 and 10intersect at a right angle. A bisector plane 11 contains the bisector ofthe angle formed by the detection fields 7 and 8 and also contains theaxis of rotation 4. The set of two detectors 1 and 2 is rotatable aboutthe axis of rotation 4 by rotating a ring 32 carried by a circular cover12. According to an essential characteristic of the invention, the set 3of detectors 1 and 2 is displaceable radially as shown by arrow 13contained in the plane 11 and intersecting the axis 4.

The assembly 3 is therefore preferably displaceable radially relative tothe cover 12. In which case the bed 6 includes lift means 14 for placingits top surface 5 at an appropriate height, together with a translationmotor for placing the portion of the patient to be investigated levelwith the pair of detectors 1 and 2. These translation movements takeplace along arrows 15 in a direction referred to as the length directionof the detection fields 7 and 8 of the detectors 1 and 2 respectively.This is the preferred solution. However, the invention produces exactlythe same results if the bed 6 is provided with means for displacing itstop surface horizontally in a direction 16 perpendicular to thetranslation direction 15 and to the lift direction 14. It can be shownthat displacements along arrows 16 in combination with displacementsalong arrow 14 are equivalent to displacing the set 3 of detectorsradially along arrow 13. The preferred solution is described below, butit should not be forgotten that the other solution is also possible.FIG. 1 also shows a swivel-mounted display console 17 having a handle 18enabling an operator to observe, inter alia, various machine settings.

FIG. 2 shows the means for displacing the set of two detectors 1 and 2relative to a patient to be examined in a direction 13 which extendsradially relative to the axis of rotation 4. This figure shows the set 3of detectors 1 and 2. It also shows the outline of the cover 12. Thecover 12 is fixed to a pedestal 19. Also supported by the pedestal 19 isa cylinder 20 of thickness e and coaxial with the cover. This cylinderis held behind the figure by the back of the cover 12 and it projectstowards the observer perpendicularly to the plane of the FIG. 2. Nearits front end, this cylinder has a circular groove 21, or optionally aprojecting rib. This rib or groove 21 guides a certain number of runningwheels such as 23 to 31 whose axles are held by the plane ring 32. Theplane of the plane ring is perpendicular to the axis of the cylinder20-21. The plane of the ring 32 lies in the plane of FIG. 2. The wheelsretain the ring 32.

A toothed groove 33 at the periphery of the ring 32 receives a drivechain 34 driven by a motor 35. The chain 34 runs round the ring 32,passes over two driving sprockets 36 and 37, and engages a controlsprocket 38. The driving sprockets are driven by the motor 35. The twodriving sprockets 36 and 37 are mounted on a plate 39 which may be movedaway from or towards a stand 40 which supports the sprocket 38. Thus byacting on actuators such as 41 and 42 it is possible to ensure that thechain 34 is properly tensioned in the groove 33 on the ring 32. When themotor 35 is caused to rotate, the ring 32 is thus driven around the axis4.

The ring 32 carries another motor 43 whose driving gear wheel 44 rotatesa gear wheel 45 whose axis is fixed substantially perpendicularlyrelative to the plane of the ring 32 (and is thus parallel to the axis4). The motor 43 is the motor that is used for moving the pair 3 ofdetectors 1 and 2 away from the axis 4. To this end, the pair 3 ofdetectors is fixed to a T-shaped bar 46. The two flanges of the T-shapedbar 46 are fixed to the edges of the pair 3, e.g. by bolts 47 to 50. Theweb 51 of the T-shape of the bar 46 lying in a plane perpendicular tothe plane of the figure is provided on either side with respective racks52 and 53. These racks mesh with respective gear wheels 54 and 55themselves driven by the gear wheel 45 via a return pulley 56 and a belt57. The belt 57 also passes between two studs 58 and 59 mountedeccentrically on a circular plate 60 for tensioning the belt 57. Whenthe circular plate 60 is rotated, the two studs 58 and 59 deform thepath followed by the belt 56 and change its tension.

When the motor 43 rotates, the T-shaped bar 46 moves up or down in thedirection of arrow 13 relative to the axis 4. The pair 3 of detectors 1and 2 is thus easily moved closer to a patient.

One of the problems to be solved with a mechanism of this kind isobtaining a corresponding displacement of a counter weight system 61 and62. Since the set of two detectors is relatively heavy, about 140 kg, itis important to balance it so that the tension exerted on the chain 34is not excessive.

FIG. 3 shows a detail of the mechanism enabling the set 3 of detectors 1and 2 to be moved synchronously with the system 61-62 of counterweights.FIG. 3 is a diagram showing the ring 32, the top of the counterweight61, and the edge 63 of the pair 3 of detectors. The counterweights 61and 62 are inside the cover 12, while the detectors 1 and 2 are outsideit. When the set 3 moves down in the direction of arrow 64, thecounterweights move symmetrically in the direction of arrow 65. A devicelike that shown in FIG. 3 is to be found at opposite ends of a diameterof the ring 32. Thus, there is a guide 66 fixed on each side of the pair3 of detectors and a guide 67 fixed to the corresponding counterweight61 (or 62). These guides are members of a generally channel-sectionshape. Each possesses a rack at the end of and perpendicular to one ofthe flanges of its channel-section shape, such as the racks 68 and 69,respectively. Toothed shafts such as 70 and 71 engage in these racks. Inpractice, there are three toothed shafts in each mechanism. Thesetoothed shafts pass through holes such as 72 formed in the ring 32. Whenthe ring is stationary, and when the set 3 of detectors is displacedusing the motor 43, the guide 66 is subjected to vertical motion of thesame size. In this case, the rack 68 meshing with the shafts 70 and 71causes them to rotate. These shafts cannot move down since they are heldvia respective ball bearings (not shown) inside the holes 72.

By reaction, these shafts move the guide 67 symmetrically by engagingwith the rack 69. The ends of the shafts 70 or 71 are fitted with guidewheels such as 73 and 74 which are held in the guides 66 and 67respectively, firstly by engaging rods such as 75 and secondly by beingheld against the rack 69 (or 68 as the case may be). By having threeshafts such as 70 and by holding the two counterweights 61 and 62together at their ends, it is possible to provide the overall assemblywith good rigidity. However, as can be seen in FIG. 2, to stabilisemotion better, the ends of the counterweights 61 and 62 may be providedwith gear wheels meshing with two racks that are likewise caused toengage a T-section bar whose web extends perpendicularly to the ring 32.

The motor 35 is the only motor that rotates the assembly, and similarlythe motor 43 is the only motor that can be used to displace thedetectors and to balance loads. The solution is therefore simple. Inpractice, the pair of detectors 1 and 2 is situated on one side of thering 32 while the set of gear wheels linked to the motor 43 is situatedinside the cover 12. Nevertheless, other similar mechanical solutionscould be devised, the essential point being that the detectors aredisplaced radially.

FIGS. 5a to 5 c give an idea of the diameter of the part that can bereconstructed depending on whether the patient is fat or thin,respectively. In each of the figures the point of intersection of theaxis 4 is shown, with the axis being in alignment with the middle of thepatient's body. In FIG. 5a, the patient is fat, the detectors thereforeneed to be moved away, and the edges 81 and 82 respectively of thefields of the detectors 1 and 2 define the circle of fullreconstruction. In one example, this circle is shown as having adiameter of 150 mm. The same items in FIGS. 5b and 5 c enable thediameters of the reconstructed space to increase with decreasing patientsize. In the invention, in order to be able to apply the reconstructionalgorithms to these diameters to be reconstructed, it is necessary toknow both the effective width 83 of the field of view FOV and theposition of the normal, e.g. 84 at effective width 83. In the invention,the position of the normal 84 and the effective width 83 are determinedby measuring the displacement of the pair 3 of detectors 1 and 2relative to a standard position.

FIG. 4 shows how the means implemented for achieving the object of theinvention are organised. Each detector, e.g. the detector 1, is providedat its inlet face 7 with a collimator 85 lying over a scintillator 86.The scintillations produced by the scintillator 86 excite the dinodes ofan array 87 of photomultiplier tubes. The currents delivered by thesephotomultiplier tubes are applied to a resistance matrix 88 whichdelivers a set of locating pulses to a counter unit 89. The counter unitis also under the control of an arithmetic and logic unit 90 via a bus91. The projection images are stored in an image memory 92 which maycontain as many pages as there are different projections acquired.

During image processing, in order to generate one or more tomographs, aprogram memory 93 delivers instructions that are performed by thearithmetic and logic unit 90 on the image signals contained in the imagememory 92. The program memory normally includes a set of parameterscontained in a parameter memory 94. In particular, this parameter memoryis shown as containing the width of the field of view and the positionof the centre of rotation.

Normally, in state of the art apparatus, the centre of rotationcorresponds to the intersection of the normals to the centres of thedetection fields 7 and 8 of the detectors 1 and 2. However, in theinvention and because of the displacements, the position of the centreof rotation must be modified by an offset δ whose value is a function ofthe value d of the displacement of the pair 3 of detectors 1 and 2. Inthe example where the detectors 1 and 2 point at substantially 90° toeach other, the relationship between δ and d is of the typeδ=d.2/2+constant. A similar trigonometrical relationship can bedetermined if the angle of inclination between the two detectors isother than 90°. However, instead of performing such calculations, aconversion table may be provided in the parameter memory enabling eachvalue 95 of the distance d to be associated with a value 96 for theposition of the centre of the effective field of view 99 and with avalue 97 representing the width of the effective field of view 99. Thesevalues 96 and 97 are then entered into the program contained in theprogram memory 93 so that the algorithm performed by the arithmetic andlogic unit 90 remains the same.

The pair 3 can be moved along the arrow 13 by means of a keyboard 98 orby some other control member such as a mouse or a track ball. Under suchcircumstances, the motor 43 may also be controlled by the microprocessorcontained in the arithmetic and logic unit 90. In one example, apotentiometer 100 may be engaged with any one of the gears linked to themotor 43. It is electrically connected firstly to a bias voltage andsecondly to ground, with its cursor giving a voltage proportional to thedisplacement that is actually performed. This voltage can be usedfirstly to servo-control the position that is to be reached and secondlyit can be used to evaluate the distance d. Alternatively, the motor 43may be a stepper type motor and measurement by means of a potentiometer100 can be eliminated merely by counting the number of steps applied tothe motor 43. The value of d is used in tables 95-97.

By acting in this way, it is shown that the invention is easilyimplemented since there is no need to change the processing program thatsuch machines already possess, while the mechanical simplification ismanifest since there is only one displacement to move both headssimultaneously.

In an improvement, the pair 3 of detectors may be caused to describe anelliptical path. In which case, each angular position α of the ring 32can be associated in advance with a distance d. This can be done in thesame way as in the correspondence tables 95-96 or 95-97. The distance dis then a function of angle α.

In the variant where the detectors are secured immobile on the ring 32and where the bed only is fitted with means for displacement followingthe arrow 13, these displacements must be carried out as a function ofthe position in rotation of the ring 32. In this case, the angle α ofthe ring 32 is measured, for example using means similar to thoserequired for measuring the rotation of the gear wheels linked to motor43. The angle α thus measured is then converted, using tables of thesame type as tables 95-96 or 95-97, into bed displacement instructionsfor following arrows 14 or 16. It can easily be seen that the movementfollowing 13 is broken down by trigonometrical functions of the firstorder (sinus or cosinus) into combined movements following 14 and 16.For a given distance d, determined for instance by instructions usingthe keyboard 98, displacements following 14 will be for example of the dsin α type, while those following 16 will be of the d cos α type.

The starting procedure is for example as follows. For α=0 (correspondingto the vertical detector 1 as in FIG. 1, for example), the bed isdisplaced following arrow 13 in response to instructions given by thekeyboard 98 and according to a size of the patient. When the desiredposition of this bed is obtained, d is validated. The value of d is setduring this validating operation, since it corresponds to a preferrednearing of the axis of the patient in the bisector plane of detectors 1and 2. The value of d is known since it corresponds to imposed andmeasurable movements of the bed following 14 and 16, in relation to acentral stopping position. During the following tomographic acquisition,and for each value of α, the motors of the bed are caused to displacethe bed according to the functions indicated above. The axis of thepatient thus describes a circular movement which is concentric to thering 12.

In a variant, the cover 12 and the ring 32 are not completely circularbut form a C-shape leaving a gap through which a an examination bed canbe moved closer laterally.

What is claimed is:
 1. Apparatus for acquiring tomographs of a subject,the apparatus comprising a pair of plane scintillation detectors (1, 2)carried by a support (32) rotatable about an axis of rotation (4), andconnected to an image processor (90-94), the apparatus beingcharacterised in that: the two detectors are secured to each other, thedetection fields (7, 8) of these two detectors are inclined at an anglerelative to each other; the bisector plane (11) bisecting the angleformed between these detection fields includes the axis of rotation; andin that the apparatus includes: means (35-42) for displacing the pair ofdetectors together relative to the subject in a direction (13) which isradial to the axis of rotation; and modification means (94) formodifying an effective detection field (98) of these detectors as afunction of said displacement.
 2. Apparatus according to claim 1,characterised in that the modification means comprise: an arithmetic andlogic unit (90) connected to: a sensor (100) for supplying data relatingto displacement of the detectors; a program memory (93) containing aprogram for computing tomographic images; and a parameter memory (94)for modifying a parameter of the image-computing program as a functionof the displacement.
 3. Apparatus according to claim 2, characterised inthe modification means comprise, connected to the arithmetic and logicunit: a rotation sensor for measuring rotation of the support; and amemory containing a path modification program for modifying thedisplacement of the detectors as a function of the rotary position ofthe support.
 4. Apparatus according to any one of claims 1 to 3,characterised in that the angle of inclination formed between thedetection fields is substantially equal to 90°.
 5. Apparatus accordingto claim 1, characterized in that the edges of the detection fields ofthe two detectors are spaced apart by less than 2 centimeters. 6.Apparatus according to claim 1, characterized in that the apparatusincludes means (65-66) for displacing detector-balancing counterweightsin the opposite direction to detector displacement.
 7. Apparatusaccording to claim 1 characterized in that the support includes acylindrical ring (32) rotatable relative to another ring (30). 8.Apparatus according to claim 1 characterized in that the supportincludes a first cylindrical ring portion rotatable relative to a secondcylindrical ring portion.
 9. Apparatus according to claim 1,characterized in that the means for displacing the pair of detectorsinclude a motor for displacing the pair of detectors relative to theirrotary support.
 10. Apparatus according to claim 1, characterized inthat the means for displacing the pair of detectors comprises a motorfor displacing a subject-carrying bed relative to the pair of detectors.11. Apparatus according to claim 1, characterized in that the twodetectors are fixed relative to each other by construction.
 12. A methodfor performing a 180° SPECT scan to form a SPECT image of an organ, theorgan being in the body of a patient oriented lengthwise along a lateralaxis, with the organ emitting gamma radiation, said method comprisingthe steps of: providing only a pair of gamma ray detectors, each havinga planar collimator surface for receiving incident gamma rays, with saiddetectors oriented so that the planar collimator surfaces aresubstantially perpendicular to a plane that is perpendicular to thelateral axis; and rotating said oriented detectors along a path toacquire image data at a plurality of positions along the path.
 13. Amethod for performing a scan to form an image of an organ in the body ofa patient, with the organ emitting gamma radiation, said methodcomprising the steps of: providing only a pair of gamma ray detectors,each having a planar detector surface for receiving incident gamma rays,with said detectors oriented so that the planar detector surfaces aresubstantially perpendicular to each other and so that the normal to eachplanar detector surface is substantially perpendicular to an axis ofrotation used in the scan; and rotating said oriented detectors toacquire image data at a plurality of positions around the patient.